The present technology relates to an extrusion coated substrate. Furthermore, it relates to the use of multi-branched polypropylene for the preparation of an extrusion coated substrate.
In the process of extrusion coating, a substrate is coated with a particular polymer so as to provide a specific functionality such as sealability to said substrate. Examples include juice and milk packings, typically having an interior polymer extrusion coated onto a foil substrate. In general, extrusion coating of substrates such as paper, paperboard, fabrics and metal foils with a thin layer of plastic is practiced on a large scale. The polymer is extruded first whereby the flux of molten polymeric material passes through a flat die to obtain a film a few microns thick, followed by a coating step, whereby the film is laid on a support and passes on a cooling cylinder. Upon cooling, the polymer adheres to its support.
The plastic most often used is low density polyethylene, a polymer which is readily extruded as a thin coating onto the surface of a moving substrate at high rates of speed. For some coating applications, crystalline polypropylene is a more desirable coating material than polyethylene due to its higher stiffness and higher heat resistance.
However, since many polypropylene materials suffer from low melt strength and low melt extensibility, they show poor processibility in high speed extrusion coating. At present, only a few polypropylene-based systems are available in the industry for extrusion coating. According to one approach for improving processibility, low density polyethylene is added to a polypropylene prepared in the presence of a Ziegler/Natta catalyst, as described e.g. in GB 992 388. In JP 2002 363356, low density polyethylene is added to a polypropylene prepared in the presence of a single site catalyst. EP-A-109 006 8 discloses a blend of a propylene homopolymer with a propylene copolymer of low crystallinity.
By blending polypropylene prepared in the presence of Ziegler/Natta catalysts or single site catalysts with either low density polyethylene or propylene copolymers of low crystallinity, processability can be improved but the level of extractables increases dramatically at moderate gel level. However, for food and beverage packaging as well as for medical packaging, high levels of extractables are not acceptable.
Furthermore, significant amounts of low density polyethylene or propylene copolymers of low crystallinity adversely affect thermal resistance as well as dimensional stability at elevated temperature. However, for many applications the extrusion coated substrate should have high thermal resistance and/or dimensional stability at elevated temperature.
According to EP-A-0947551, processibilty is improved by post-reactor modification, such as treatment by irradiation or free radicals. However, although known post-reactor modification processes can improve processibility, they result in a high level of extractables. Furthermore, the gel-rating of post-reactor modified resins is typically high.